Radio communication systems such as mobile telephone systems and wireless LANs (Local Area Networks) that have appeared in recent years use modulation formats such as QPSK (Quadrature Phase Shift Keying) and multi-value QAM (Quadrature Amplitude Modulation). In these modulation formats, when a signal changes between symbols, since its waveform is involved in amplitude modulation, the amplitude (envelop) of a radio frequency modulation signal superimposed with a carrier signal of a microwave bandwidth varies with time. The ratio of the peak power and the average power of the radio frequency modulation signal is referred to as PAPR (Peak-to-Average Power Ratio). When a signal having a large PAPR is amplified with a high linearity maintained, a power supply device needs to supply sufficiently large power to an amplifier so as to prevent the waveform of the amplified signal from being distorted with peak power and to secure high linearity of the amplified signal. In other words, the amplifier needs to be operated in a back-off power region that is sufficiently lower than the saturated output power restricted by the power supply volte.
Generally, since radio frequency amplifiers that amplify a radio frequency signal according to the class A system or class AB system have a maximum efficiency that is nearly at the saturated output power level, if they are operated in the large back-off power region, their average efficiency lowers.
Although the OFDM (Orthogonal Frequency Division Multiplexing) system has been used in the next-generation mobile phone systems, wireless LANs, digital television broadcasts and so forth, since their PAPR tends to increase, the average efficiency of radio frequency amplifiers further lowers. Thus, it is desired that radio frequency amplifiers operate with high efficiency even in the large back-off power region.
As a system that amplifies a signal in the back-off region and with wide dynamic range and high efficiency, a power amplifying device referred to as the EER (Envelope Elimination and Restoration) system has been proposed in Non-patent Document 1.
The EER system proposed in Non-patent Document 1 divides an input modulation signal into a phase modulation component and an amplitude modulation component. Thereafter, the phase modulation component with a constant amplitude is inputted to the amplifier in such a manner that phase modulation information is maintained. At this point, the amplifier is always operated at nearly the saturated output power level that provides the maximum efficiency.
On the other hand, the amplitude modulation component is amplified with high efficiency by a class D amplifier or the like in such a manner that amplitude modulation information is maintained and then supplied as a power supply voltage whose output intensity has been modulated (modulation power supply).
When the power amplifying device is operated in such a manner, the radio frequency device is also operated as a multiplier and combines the phase modulation component and amplitude modulation component of the modulation signal and outputs the combined result. Thus, the radio frequency amplifier can obtain an output modulation signal amplified with high efficiency not in the large back-off power region.
As a system similar to the EER system, the so-called ET (Envelop Tracking) system is also known. This system has been reported for example in Non-patent Document 2 and so forth.
The ET system and the EER system have the same structure that uses a class D amplifier or the like that amplifies the amplitude modulation component of the modulation signal in such a manner that the amplification modulation information is maintained and supplies the resultant signal as a power supply voltage (modulation power supply) whose output intensity has been modulated to the amplifier.
The EER system and the ET system have the same structure except that, in the EER system, the former inputs only a phase modulation signal with a constant amplitude to the amplifier so as to operate it at nearly a saturation output power level; in the ET system, while the latter inputs an input modulation signal containing both amplitude modulation component and phase modulation component to the amplifier so as to linearly operate it.
Although the ET system is inferior to the EER system in efficiency because the amplifier of the former operates linearly, since only bare minimum power based on the amplitude modulation component of the input modulation signal is supplied to the amplifier, the former can have higher efficiency than the structure that supplies a constant power voltage to the amplifier.
In addition, the ET system can be more easily realized than the EER system because the former allows a timing margin at which the amplitude modulation component and the phase modulation component to be combined to be loose.
The EER system and the ET system generally use a modulation power supply that converts the amplitude modulation component into a pulse modulation signal and performs switching amplification for the pulse modulation signal using a class D amplifier or the like. As a pulse modulation system for the EER and the ET system, the PWM (Pulse Width Modulation) system has been traditionally used; however, Patent Document 1 and Patent Document 2 propose structures that use the delta modulation system (or PDM (Pulse Density Modulation) that has higher linearity than the foregoing systems. Moreover, in recent years, the sigma delta modulation system and so forth that have a high SNR (Signal to Noise Ratio) have been used as pulse modulation systems.
Relevant standards for radio communication systems such as mobile telephone systems and wireless LANs using digital modulation systems that have appeared in recent years require that ACPR (Adjacent Channel Leakage Power Ratio) and EVM (Error Vector Magnitude) should be suppressed to a predetermined constant value or below.
To satisfy these standards for power amplifying devices according to the EER system and ET system, it is said that the bandwidth in which a pulse modulator and a class D amplifier with which a modulation power supply is provided needs to be at least twice the bandwidth of a modulation signal. For example, the modulation bandwidth of WCDMA (Wideband Code Division Multiple Access) used in mobile phone systems is around 5 MHz, whereas the modulation bandwidth of IEEE 802.11a/g used in wireless LANs is around 20 MHz. Generally, it is difficult to switch a large power at high speed and realize a modulation power supply that operates in such a wide bandwidth.
A power amplifying device provided with a modulation power supply having the simplest structure has been proposed in Patent Document 3. FIG. 1 shows a structure of a power amplifying device presented in Patent Document 3 (this device is hereinafter referred to as the first related art reference).
The power amplifying device according to the first related art reference has a structure that supplies an average power (power supply voltage) to an amplifier in an ordinary state and supplies a large power (power supply voltage) to the amplifier only when the amplitude value reaches a constant value or greater.
With reference to FIG. 1 and FIG. 2, an operation of the power amplifying device according to the first related art reference will be described.
The power amplifying device according to the first related art reference ordinarily supplies voltage Bc as a power supply voltage to amplifier 204 (refer to FIG. 2 (c)).
Voltage Bc is ordinarily designated to be lower than the maximum output voltage so as to obtain an average output power. When envelop sensor 201 detects a peak at which envelop (amplitude modulation component) 9 of the input modulation signal becomes greater than reference voltage Vref (FIG. 2 (a)), envelop sensor 201 outputs control signal 10 (FIG. 2 (b)).
Power bulb 203 is turned on based on control signal 10 and then voltage 11 to which maximum voltage Bv has been added is applied to amplifier 204 (FIG. 2 c). The power bulb having a structure using capacitance coupling has been proposed in Patent Document 4; a bulb having a structure using both capacitance coupling and magnetic coupling has been proposed in Patent Document 5.
Since these structures do not allow wasteful power to be supplied to amplifier 204 in a region where the amplitude modulation component of the modulation signal is small, the average efficiency of amplifier 204 can be improved.
Another structure of a modulation power supply that operates with high efficiency and a wide bandwidth has been proposed in Non-patent Document 3. The structure of the power amplifying device proposed in Non-patent Document 3 (this device is hereinafter referred to as the second related art reference) is shown in FIG. 3.
The power amplifying device according to the second related art reference interlocks linear amplifying section 3 that operates in a wide bandwidth, but with low efficiency and switching regulator section 2 that operates in a narrow bandwidth, but with high efficiency so as to supply modulation power (power supply voltage) 11 with high efficiency in a wide bandwidth to amplifier 1. With reference to FIG. 4, a specific operation of the power amplifying device will be described.
Inputted to linear amplifying section 3 that is composed of a differential amplifier and that operates as a voltage follower is amplitude signal 9 that is the amplitude modulation component of modulation signal 8. In this example, it is assumed that amplitude signal 9 is a 2 MHz sine wave (FIG. 4 (c), Reference numeral 9).
An output current of linear amplifying section 3 is converted into a voltage signal by current detection resistor 42 and inputted to hysteresis comparator 41. In this example, if polarities are selected such that when the current flows from linear amplifying section 3, the output voltage of hysteresis comparator 41 becomes High and when the current flows to linear amplifying section 3, the output voltage of hysteresis comparator 41 becomes Low, a pulse width modulation signal based on the output signal of linear amplifying section 3 is outputted from hysteresis comparator 41 (FIG. 4 (c), Reference numeral 10).
Gate driver 5 turns on or off switching device 21 composed of for example an MOS FET (Metal Oxide Semiconductor Field Effect Transistor) based on the output signal of hysteresis comparator 41. Switching device 21 composes switching regulator section 2 in combination with diode 22; switching regulator section 2 amplifies the amplitude of the pulse width modulation signal to Vcc1.
The pulse width modulation signal that has been amplified is integrated by inductor 6 and thereby the switching frequency component is removed therefrom (FIG. 4 (a)).
An error component contained in an output current of inductor 6 is compensated by linear amplifying section 3 and supplied as a power supply voltage to radio frequency amplifier 1. At this point, since a current (FIG. 4 (b)) that flows in linear amplifier 31 with low efficiency contains only the error component, the power consumed in linear amplifier 31 is small and most of the signal components of amplitude signal 9 are amplified by switching regulator section 2 with high efficiency. Thus, the efficiency of the entire power amplification device can be improved.
However, since the power amplifying device according to the first related art reference of those presented above needs to operate in a large-back-off voltage region such that the output amplitude of amplifier 204 always becomes lower than modulation voltage 11 (FIG. 2 (c)), as a problem of the first related art reference, the effect of improved efficiency is low. In addition, since voltage waveform 111 that has been modulated becomes in a hard clipping shape, as a problem of the first related art reference, the output spectrum deteriorates.
On the other hand, in the power amplifying device according to the second related art reference, since linear amplifier 31 compensates the voltage in such a manner that voltage waveform 11 supplied to radio frequency amplifier 1 becomes close to the waveform of amplitude signal 9, the power amplifying device according to the second related art reference improves the efficiency and suppresses deterioration of the spectrum in comparison with the power amplifying device according to the first related art reference.
However, if the power amplifying device according to the second related art reference is used for a device having a high transmission power such as a radio base station of a mobile phone system, since power supply voltage Vcc1 becomes around 28V, in order to turn on switching device (ordinarily, an FET) 21, it is necessary to raise the amplitude of output signal 10 of gate driver 5 to Vcc1 or greater. Such a structure can be ordinarily accomplished by using a boot strap circuit or the like for gate driver 5, and generally it is difficult to operate such pulses having a large amplitude at high speed.
Thus, in the power amplifying device according to the second related art reference, as shown in FIG. 4 (c), the switching frequency is restricted to a low value and thereby switching regulator section 2 can amplify only signal components in a bandwidth from DC to around 100 kHz. Thus, since signal components in higher bandwidths are amplified only by linear amplifier 31 with low efficiency, as a problem of the second related art reference, the efficiency of the entire power amplifying device deteriorates.
Moreover, in the systems that modulate a power supply voltage of an amplifier such as the foregoing EER system and ET system, when the value of the power supply voltage (voltage waveform 11) that is supplied from the modulation power supply to amplifier 1 gets close to 0, since the gain of the amplifier becomes small, as a problem of the second related art reference, the waveform of output signal 12 distorts. Thus, it is preferred that the output voltage of the modulation power supply be designated a constant lower limit value (DC offset).
In the power amplifying device according to the second related art reference, although switching regulator section 2 amplifies a signal including a DC offset, if it operates with a large power in a high speed switching state, it is difficult to allow the switching regulator to obtain an efficiency of 90% or greater.